1. Field of the Invention
This invention relates to reusable hot melt adhesive or coating compositions comprising mixtures of resins, waxes, and tackifiers. The compositions melt at significantly lower temperatures than conventional hot melt adhesives or coatings and may be used to permanently or temporarily bind or coat industrial substrates during manufacturing processes. This invention especially relates to use of the novel hot melt adhesive compositions for temporarily holding ophthalmic plastic lenses to supporting metal blocks during grinding and polishing of lenses.
2. Description of the Related Art
Indium alloy adhesive has long been used in the ophthalmic lens manufacturing industry for securing lenses to substrates during grinding. Typically the convex side of the lens is positioned on a block using a gasket. Indium metal is applied in the cavity. After the indium hardens, the lens is ground and then mechanically removed by applying mechanical force to the lens while holding the supporting block stationary. The use of indium has several problems associated with it, some of which include worker toxicity and environmental disposal. As a result, the ophthalmic lens manufacturing industry has searched for a suitable adhesive for holding ophthalmic plastic lenses to supporting substrates during grinding and polishing of the lenses.
Application of heat to a hot melt adhesive brings it to a liquid state, and after removal of the heat it sets by cooling. Where the cooling is rapid, near instantaneous bonding is possible. A hot melt adhesive system frequently offers increased production speeds and lower costs than other adhesive systems.
In general, useful hot melt adhesive compositions are essentially thermoplastic. In the past, a large number of natural and synthetic waxes and resins have served as principal constituents in various useful hot melt adhesive compositions. In addition, tackifiers have been used to modify the properties of the compositions.
In general, hot melt adhesives have not been used for the ophthalmic lens application because the adhesives have required higher melting temperatures than may be used on ophthalmic plastic lenses. Normally, it is not possible to use any adhesive that will contact the lens at a temperature higher than 190.degree. F. (about 88.degree. C.). For most practical applications, it is better if the adhesive contacts the plastic lenses at a temperature no higher than 175.degree. F. (about 80.degree. C.). If the adhesive is applied at a higher temperature, it may result in distortion of the lenses.
Because of the nature of the plastic lens grinding process, it is desirable to have a lens adhesive with a low softening point, adequate adhesion to both the supporting block and the plastic lens, sufficient rigidity to prevent rocking of the lens during the grinding operation, and a low melt viscosity for easy application. In addition, the adhesive should be mechanically removable from lenses, without leaving a residue.